use new orientation for magnetic models (Trac #910)

[pkienzle]

theta and phi for magnetism direction do not operate like theta and phi. Consider changing to a latitude/longitude coordinate system. Make sure it is clear from the docs.

Migrated from http://trac.sasview.org/ticket/910

{
    "status": "new",
    "changetime": "2018-04-06T18:37:16",
    "_ts": "2018-04-06 18:37:16.507249+00:00",
    "description": "theta and phi for magnetism direction do not operate like theta and phi.  Consider changing to a latitude/longitude coordinate system.  Make sure it is clear from the docs.",
    "reporter": "pkienzle",
    "cc": "",
    "resolution": "",
    "workpackage": "SasView Bug Fixing",
    "time": "2017-04-03T21:01:41",
    "component": "SasView",
    "summary": "use new orientation for magnetic models",
    "priority": "major",
    "keywords": "",
    "milestone": "SasView 4.3.0",
    "owner": "",
    "type": "defect"
}

[butlerpd]

Trac update at 2017/10/27 15:55:02: butler changed milestone from "SasView 4.2.0" to "SasView 4.3.0"

[pkienzle]

Trac update at 2018/04/06 18:37:16: pkienzle commented:

Setting the magnetic orientation relative to the particle would be more convenient than setting it relative to the lab frame.

If magnetic orientation is relative to the particle, then particle orientation distribution should lead to a magnetic orientation distribution. However, symmetric particles would then need psi and possibly theta and phi orientations to control the distribution.

Magnetic saturation leads to orientation in the lab frame. This can be done by tying magnetic orientation to negative particle orientation, but it will not work correctly with orientation distribution on the particles.

[dehoni]

The ticket has a documentation defining magnetisation vector and polarisation in line with particle orientation notation (code change is still needed). This is the most general description of magnetic SANS ever written. It will even allow for Spherical Polarimetry in SANS. It gives more freedom in field/polarisation axis away from the detector plane, e.g. to put it in-beam direction,
i.e. allow in-beam direction or oscillatory/rotational fields...). With proper generalised orientation distribution, you cover all "normal" use cases.
You could couple particle magnetisation with particle orientation or the lot of it relative to the field axis, but will get easily quite messy and interdependent. I would not consider this. Anyway this can be set by constraints in the model by the user, if wanted.

[dehoni]

In the changes for ticket 432 also the magnetization and polarization direction is harmonized with the particle orientation.